The invention concerns a flow-through microfluidic nuclear magnetic resonance (=NMR)-chip comprising a substrate which is planar in an yz-plane with a sample chamber within the substrate, the sample chamber being elongated and having walls which run parallel to the z-direction the substrate having a thickness in x-direction of a Cartesian xyz-coordinate system between 50 μm and 2 mm, and at least one planar receiving and/or transmission coil with conductor sections the coil being arranged at least on one planar surface of the substrate, wherein the extension of the sample chamber along the z-direction exceeds the extension of the coil along the z-direction.
Such a flow-through microfluidic NMR-chip is known from [6].
Small-volume samples for nuclear magnetic resonance (NMR) spectroscopy consist for example of less than a few microliters of an analyte in solution. Such small samples are conveniently handled using microfluidic channels fabricated in different types of substrates, together with valves, pumps, and other miniaturized sample preparation and transportation means. NMR analysis in so-called lab-on-a-chip or micro total analysis systems (microTAS) is of great interest. However, because NMR has inherently a low sensitivity and since the NMR signal is proportional to the sample volume, signal-to-noise ratio becomes very weak for such small samples. One of the means to increase the sensitivity of the NMR experiment for small-volume sample is the use of miniaturized radio frequency (=RF) coils for signal detection. Coils which show dimensions of a few millimeters down to tens of micrometers can be fabricated by photolithography directly on the microfluidic substrate. Nevertheless, the quality of NMR spectra obtained so far from samples contained in microfluidic NMR-chips has been rather poor and appropriate coil-sample configurations remain to be designed in order to improve the NMR detection performance.
In [1] an NMR apparatus is disclosed wherein a planar, lithographic microcoil is fabricated onto a substrate onto which has been etched or grooved channels to serve as capillaries through which analytical compound flows. [2] describes an integrated miniaturized device for processing and NMR detection of liquid phase samples. Essential performance criteria of an NMR probe are its spectral resolution, sensitivity and homogeneity of the RF field of the microcoil (=B1 homogeneity). While the NMR detection performance is largely determined by the NMR coil—sample configuration, none of the above mentioned documents describes an arrangement yielding good NMR performance. In fact, the NMR performance of planar microcoils to date has been rather poor, in particular with respect to spectral resolution.
In [3] a rectangular Helmholtz coil geometry with the aim of improving B1 homogeneity is proposed, as shown in FIG. 17c. The coil 102 was combined with a rectangular closed cavity 202 of dimensions smaller than the dimensions of the coil 102 for sample containment. The authors acquired a proton spectrum of a vinyl plastisol material at 63.5 MHz, which resulted in a very broad spectrum.
In [4] a flow-through microfluidic NMR-chip is disclosed with an enlarged sample chamber and a planar circular coil. A proton spectrum of sucrose obtained from that prior art microfluidic NMR-chip configuration is illustrated in FIG. 18a. The spectrum was acquired at 300 MHz with a 1M sucrose sample concentration in D2O. The microcoil was a two-turn circular spiral with an inner diameter of 2 mm. The active sample volume was 470 nL. The measured linewidth was 20 Hz and splitting of the anomeric proton peak could not be observed. B1 homogeneity performance of the same prior art microfluidic NMR-chip configuration is illustrated in FIG. 18b. The signal amplitudes measured for a 270° was 54% of that obtained for a 90° pulse. Typical specifications for a conventional NMR probe require at least 50% signal amplitude for an 810° pulse. This prior art configuration was far from meeting these specifications.
Walton et al. [5] used a circular coil geometry 100 in combination with a spherical sample chamber 200 (FIG. 17a). The authors did not acquire 1H spectra, but derived qualitative proton line widths on the order of 20 Hz (at 300 MHz) from 31P measurements. It is pointed out that standard shim coils were ineffective in further improving the resolution. With a linewidth of 20 Hz, fine features of proton spectra (such as J-coupling) cannot be resolved. An additional drawback of their design is a low sensitivity due to a small filling-factor.
In order to improve spectral resolution, the configuration illustrated in FIG. 17b was proposed by Wensink et al. [6]. It consists of a straight channel 201 in line with the static magnetic field combined with a circular microcoil 101 placed in the central region of the channel. With this configuration, the authors measured a linewidth of 1.3 Hz at 60 MHz. This value would correspond to a proton linewidth of 6.5 Hz at 300 MHz. This is still not adequate to perform high-resolution NMR spectroscopy. It can be shown that such a configuration would yield poor B1 homogeneity performance.
In summary, none of the prior art configurations simultaneously yielded a good spectral resolution, high sensitivity and large B1 homogeneity.
It is therefore an object of the invention to suggest a flow-through microfluidic NMR-chip with improved resolution, sensitivity as well as B1 homogeneity in order to achieve good NMR performance within microfluidic NMR-chips.